A communication scheme subsequent to wideband code division multiple access (W-CDMA) or high speed downlink packet access (HSDPA), that is, long-term evolution (LTE) has been established by a 3rd generation partnership project (3GPP) standardization group. In LTE, as a radio access scheme, orthogonal frequency division multiple access (OFDMA) is employed in downlink, and single-carrier frequency division multiple access (SC-FDMA) is employed in uplink.
In OFDMA, a frequency band is divided into a plurality of narrow frequency bands (subcarriers), and data are conveyed on each frequency band for transmission. Fast transmission is implemented by densely arranging subcarriers over frequencies without interference while they partially overlap with one another, so that it is possible to increase spectral efficiency.
In SC-FDMA, it is possible to alleviate interference between terminals by dividing the frequency band and transmitting signals using different frequency bands between a plurality of terminals. In SC-FDMA, since transmission power fluctuation is reduced, it is possible to reduce power consumptions in terminals and implement wide coverage.
The LTE system performs communication by causing a plurality of mobile stations (user equipment: UE) to share one or more physical channels in both uplink and downlink. The channel shared by a plurality of the mobile stations UEs is generally called a shared channel. In LTE, the shared channel is referred to as a physical uplink shared channel (PUSCH) in uplink and as a physical downlink shared channel (PDSCH) in downlink.
In the communication system using the shared channel described above, it is necessary to perform signaling regarding which mobile station UE the shared channel is allocated to in every subframe which is a unit of transmission time. The subframe may be referred as a transmission time interval (TTI).
In LTE, a physical downlink control channel (PDCCH) is defined as a downlink control channel used in the signaling described above. Furthermore, a physical control format indicator channel (PCFICH) is defined as a control channel for notifying the number of OFDM symbols used in PDCCH, and a physical hybrid-ARQ indicator channel (PHICH) is defined as a control channel for transmitting ACK/NACK information of the hybrid-ARQ for PUSCH.
The downlink control information carried on PDCCH includes, for example, downlink scheduling information, an uplink scheduling grant, an overload indicator, and a transmission power control command bit (Non-patent Literature 1). In addition, the downlink scheduling information includes, for example, downlink resource block allocation information, an identification of UE, the number of streams, information on precoding vectors, a data size, a modulation scheme, and information on a hybrid automatic repeat request (HARQ). In addition, the uplink scheduling grant includes, for example, uplink resource block allocation information, an identification of UE, a data size, a modulation scheme, uplink transmission power information, and information on a demodulation reference signal.
In the PCFICH, the number of OFDM symbols for PDCCH mapping is notified as control channel format information (control channel format indicator: CFI). In LTE, the number of OFDM symbols for PDCCH mapping is set to 1, 2, or 3. In addition, PDCCH is mapped from the leading end of the OFDM symbols in a single subframe (Non-patent Literature 2).
In downlink, a range corresponding to the CFI (number of OFDM symbols) notified on PCFICH from the leading end of the subframe becomes a control channel area allocated to PDCCH. The mobile station decodes the control channel area and further decodes a radio resource allocated to PDSCH based on the downlink control information if there is information destined to the mobile station itself.